1. Field of the Invention
The present invention relates to a bias circuit for a photodiode and, more particularly, to a bias circuit for a photodiode, which applies a reverse voltage to the photodiode.
2. Related Background Art
In a device for detecting a photoelectric current according to a radiation light amount using a photodiode, as shown in FIG. 1, a photodiode 1 is used in a state wherein a potential across an anode-cathode path of the photodiode 1 is set in a zero or reverse bias state by a power supply 5 (power supply voltage V.sub.R). More specifically, when the linearity of a photoelectric current output in a low-illuminance range based on a photovoltaic effect is to be assured, V.sub.R =0 is set to use the photodiode 1 in a zero bias state, thereby eliminating dark current components and improving the S/N ratio. On the other hand, when high-speed response characteristics are required, V.sub.R &gt;0 is set to apply a reverse bias across the anode-cathode path of the photodiode 1. In this manner, the response characteristics are improved by widening the depletion layer, decreasing the junction capacitance, and strengthening the depletion layer electric field.
As a means for current-voltage converting a photoelectric current, the following method is popular. That is, as shown in FIG. 2, the photodiode 1 is connected to one input terminal (inverting input terminal in FIG. 2) of an operational amplifier 8, and the output from the operational amplifier 8 is fed back to the input terminal (inverting input terminal) of the operational amplifier 8 via a current-voltage conversion negative feedback element 9. At this time, one of the anode and cathode terminals of the photodiode 1 is connected to a power supply 6 having a predetermined potential (V.sub.K), and the other is fixed to a power supply 7 having a potential (V.sub.A) applied to the other input terminal (non-inverting input terminal in FIG. 2) of the operational amplifier 8 by virtual grounding across the two input terminals of the operational amplifier 8. Based on the potential relationship between the anode and cathode, the photodiode 1 is biased by a reverse voltage given by V.sub.R =V.sub.K -V.sub.A.
As a means for applying a reverse voltage across the anode-cathode path of the photodiode 1, a means using an external power supply is also available. However, in general, as shown in FIG. 3 or 4, a means for stacking a potential at one main voltage terminal upon combination of constant voltage elements such as diodes and a load element such as a resistor is adopted. More specifically, in FIGS. 3 and 4, the cathode potential V.sub.K of the photodiode 1 is clamped at a potential of a total of five diodes as follows: EQU V.sub.K =5.times.V.sub.BE .perspectiveto.3.5 V
In this case, since the anode potential is grounded, a reverse voltage given by the following equation is consequently applied to the photodiode 1: EQU V.sub.R =V.sub.K
However, in the prior arts shown in FIGS. 3 and 4, since the potential across the anode-cathode path of the photodiode 1 is determined by the constant voltage elements and the load element inserted between main voltage terminals, if the main voltage terminal defining the potential drifts in an AC manner due to the influence of, e.g., noise, a current flowing through the main voltage terminal changes. For this reason, a reverse bias amount across the anode-cathode path of the photodiode 1 varies, and an AC current corresponding to this variation flows through a junction capacitance 3 of the photodiode 1, thus erroneously operating the circuit.
More specifically, in, e.g., FIG. 3, when the light amount is 0 (photoelectric current .perspectiveto.0), a current i.sub.1 flowing through the diodes D5 to D9 is given by: When V.sub.CC drifts, i.sub.1 changes to satisfy: ##EQU1##
The potential of each diode is changed by: ##EQU2##
As a result, the reverse bias amount V.sub.R of the photodiode 1 drifts to satisfy: ##EQU3##
With this drift, a current given by the following equation flows through the junction capacitance 3 of the photodiode 1: ##EQU4##
Thus, a wrong signal is detected by a current detector 2.
This problem can be solved by determining the potential across the anode-cathode path of the photodiode 1 using a constant voltage circuit having a good PSRR (power supply reduction ratio). However, the circuit itself then becomes a complicated and high-grade one, and the number of elements is increased, resulting in an increase in cost.